Patent GB-1297406-A (GB)
IPC: G01S15/60
Filed: 1970-06-16
Abstract: 1297406 Doppler navigation; frequency multipliers INSTITUT FRANCAIS DU PETROLE DES CARBURANTS ET LUBRIFIANTS 16 June 1970 [20 June 1969] 29160/70 Headings H3P and H4D A method of Doppler navigation includes transmitting a whole number of cycles of acoustic energy intermittently along at least one direction oblique to a reference direction, receiving reflected signals and measuring the number of integral cycles received during a time #<SP>1</SP> less than the received pulse length and calculating digitally therefrom, the distance travelled in the recurrence period of the intermittent signals. Three transducers 5a, 5b, 5c, on a ship direct pulsed acoustic energy along the edges of a square based pyramid and receive backscattered energy from the sea-bed. By combining the signals from one transducer 5a with each of the other two, the movement of the ship along two orthogonal axes is measured. Choosing suitable parameters for the pyramid the distance travelled is equal to where # 0  is the wavelength of the transmitted energy, N is the number of transmitter cycles in a transmitter recurrence period, N 3 Œ# is the number of cycles in the received signal in a time #<SP>1</SP> and N 2  is the number of transmitter cycles in a time #<SP>1</SP> (Fig. 1, not shown). An oscillator in unit 1, Fig. 5, produces a signal of frequency f 0  which is pulse modulated and fed to three piezo-electric transducers 5a, 5b, 5c. Reflections from the sea-bed are picked up thereby and passed to three band-pass amplifiers 6a, 6b, 6c shown in Fig. 6. The transducers are connected to the primary of transformer 20 whose secondary is connected to a voltage divider comprising resistor 21 and diodes 22a, 22b. When a pulse in transmitted bistable 23 forward biases the diodes such that the input to amplifier 24 in very low. After transmission the diodes are reverse-biased allowing the received signals to be clipped and passed through amplifier 24 to two active filters which together form a band-pass filter. The output from the reference axis amplifier 6b, Fig. 5, in then mixed with a signal, f 0 , and the low frequency component removed leaving a signal 2f 0 +f 1 , where f 1  is the Doppler shift along the reference axis. This signal is used as one input to four demodulators 10a-10d whose second inputs are from the other two channels direct or through #/2 delays 8, 9. The outputs from these four demodulators are passed to frequency multipliers 11a-11d, where the frequency is multiplied by 2<SP>n</SP>, to reduce the error due to fractions of cycles, received in time #<SP>1</SP> by a factor of 2<SP>n</SP>. The input to the multiplier (Fig. 8, not shown) is passed to two parallel limiting amplifiers (36, 37) one of which (37) is also an integrator. Both complementary outputs of the amplifiers are differentiated, rectified, added and applied to the input of an amplifier (40) which feeds a bistable (41) whose output is thus at twice the input frequency. A chain of n such stages produces the desired result. After this multiplication the signals are mixed with a signal of frequency 2<SP>n</SP>f 0  producing two signals at each frequency, 2<SP>n</SP>(f 1 -f 2 ) and 2<SP>n</SP>(f 1 -f 3 ). The pairs of signals at each frequency are applied to limiting amplifiers 13a-13d producing digital signals therefrom and are then fed to units 14a, 14b which produce pulses at outputs C, D respectively if f 1  is greater or less than the other frequency f 2  or f 3 , thus determining the sense of movement of the ship. Amplifier 13a is fed with a signal representing cos 2<SP>n</SP>(f 1 -f 2 )2#t and 13c with the sine equivalent. The positive and negative squared outputs from amplifier 13c are differentiated and compaired in two AND gates with the positive output from 13a. Since sin #t = -sin(-#t), the AND gates will produce outputs representing one pulse per cycle of the input and the particular and gate which supplies the output indicates whether f 1 -f 2  is greater or less than zero, i.e. the direction of travel of the ship. The pulses from the C and D output terminals are fed to computers 16a, 16b shown in Fig. 10. If the first pulse arrives at C during time #<SP>1</SP>, flip-flop 47 is sot through gate 59 such that Q = 0, Q = 1 thus, the pulse passes through gates 53a, 55, 57 to the adding input C of counters 48, 49. The contents of counter 49 is read into register 50 through gate 61. Since the content is now non- zero, gate 64 is closed holding flip-flop 47 in that position. Thus subsequent pulses arriving at terminals C, D are applied to the C and D inputs respectively of the counters which thus measure N C -N D . If, however the first pulse arrives at D, flip-flop 47 is held in the opposite state and the pulses are applied to the opposite inputs of the counters which thus measure N c -N n . At the end of the time #<SP>1</SP>, clock pulses applied through gate 62 shift the register 50 content by p places, i.e. multiplying by 2<SP>P</SP>. The register is then emptied through gate 66 or 67 depending on the relevant direction of travel sensed by unit 14 and selected by flip-flop 47. A resetting pulse RAZ from a synchronizing unit then resets the registers to zero and the process is repeated for the next received pulse. The outputs from computers 16a, 16b are thus a measure of the distance moved in two orthogonal directions (since 2P<SP>+n</SP> = N/N 2 ) in terms of # 0 . Unit selectors 17a, 17b are used to select the display in counters 19a, 19b to be in terms of # 0 , meters or fathoms. The selection is accomplished by a three-way switch to pass the computer output direct to the display or to introduce digital counters to divide by the appropriate factors. In synchronizer 1, Fig. 11 oscillator 68 provides a signal of frequency 2<SP>n</SP>f 0  which is fed to the mixers (12a-12d, Fig. 5) and also to counters 69, 71, 72 which respectively produce pulses at frequency f 0  and at times N 1 t 0  and Nt 0 . The signal of frequency f 0  passes AND gate 73 enabled by gate 79 fed with signals from all parts of the system indicating correct functioning thereof. The first pulse enable bistable 75 and produces pulses indicating t 1  and the reset pulse RAZ. Counter 74 produces an output at t 2  = N 2 T 0  which resets, the bistable 75 whose output thus indicates #<SP>1</SP>. A further pulse from counter 74 at time t 2 +pT 0  is used to initiate read out for the count in register 50, Fig. 10. A similar counter 77 provides synchronzing pulses for the second channel indicating movement in the orthogonal direction.